The ability to measure the blood electrolytes and blood gases on demand, at bedside, greatly enhances the successful management of the critically ill patient. It is common practice to make in vitro (stat) measurements of the blood electrolytes Na, K, and Ca, and blood gases including pO2, pCO2 and pH in a central or satellite laboratory. During critical stages of the patient, while in surgery, the emergency room or the intensive care unit, the physician has frequent need for the patient's electrolyte and blood gas status. Such a request is made by calling the central or satellite laboratory. The laboratory will send a technologist to collect two blood samples from the patient. These samples are collected anaerobically, placed in an ice bath, and taken to the laboratory. Upon arrival in the laboratory, the samples are entered in the log book and subjected to sample preparation, which includes such procedures as centrifugation and dilution of the serum, so obtained, for analysis.
The analysis of the electrolytes and the blood gases is done according to two distinct procedures and on two separate instruments. For blood gas analysis, an aliquot of whole blood is drawn from one of the samples and introduced into a blood gas analyzer. Any transfer of blood sample must follow careful anaerobic techniques to prevent shifting of the blood gas values due to sample handling procedures. Equally critical is the calibration procedure of the pO2, pCO2 and pH electrodes for the blood gas measurement. These electrodes are calibrated on a tonometer, where they are exposed to a liquid and/or gas phase, having a known partial pressure of O2 and CO2. This process is time consuming. Electrolytes are measured with an instrument which may use ion selective electrodes, flame photometry or atomic absorption techniques. Considering the sampling, sample preparation, calibration, measurement, and logging of the results, the physician may have to wait fifteen to forty-five minutes to get the values for a diagnostic decision. During critical care, shifts in electrolyte and blood gas values can happen rapidly, as a result of or resulting into significant physiologic events, which may endanger the patient. It is for these reasons that the ability to make bedside measurements of the blood electrolytes and blood gases, on demand, will significantly contribute to the successful management of the critically ill.
One form of in-line measurement of the blood electrolytes reported in the literature is to continuously remove a small volume of blood from the patient. The point of blood withdrawal may be a connector in a by-pass line between the patient and the heart-lung machine, or an intravenous line directly from the patient. A pump controlled test sequence alternately exposes the measuring electrodes to calibrant solution, blood and flushing solution. In this system the blood, after being measured, is discarded and not returned to the patient. This system also requires that the sampling line at the point of entry to the patient's blood stream is regularly flushed with a heparinized saline solution to prevent clot formation. The process of clot prevention is accomplished by flushings, in-line calibration, and measurement is complex.
The measurement of the chemistry of other body fluids, principally urine, also has been slow and inconvenient and costly. Even less progress has been made in the prior art to a solution to the problem of measuring the chemistry of these fluids.